The invention relates generally to the field of hydraulics on refuse trucks and more particularly to a hydraulic pump unloading or control system on refuse trucks. The system may have a plurality of fixed displacement pumps driven by a single engine and a valve incorporating electrical control circuitry for selectively activating diverse combinations of pumps in response to drive conditions such as engine speed, the circuit being activated, and the pump pressure during activation.
Many hydraulic circuit configurations and combinations of valves have been devised with the purpose of modifying the hydraulic pressure or flow supply based on the power requirements of the system or based on the availability of power to drive the system. A majority of such systems use variable displacement pumps that are typically more expensive and complex than standard fixed displacement pumps. However, there are some systems that use fixed displacement pumps in combination with hydraulic control circuitry to accommodate variability in hydraulic pressure and flow supply. For example, U.S. Pat. No. 4,164,119 provides an unloading system with fixed displacement pumps that prevents stalling of the drive engine in response to flow or pressure conditions in the hydraulic line. The multiple fixed displacement pump system of U.S. Pat. No. 4,002,027 modifies flow supply by combining two pump outputs when necessary based on pressure or flow conditions in the hydraulic lines with the aim of eliminating the need for high engine and pump speed solely to supply flow requirements. Yet another system in U.S. Pat. No. 4,381,904 provides a circuit with numerous fixed displacement pumps selectively activated in response to pressure and flow conditions in the hydraulic line as a means of providing variable pressure and flow requirements.
The prior art in hydraulic pump unloading or control systems with a plurality of fixed displacement pumps has heretofore used pressure and/or flow response means to drive the logic of the variable flow and pressure supply. Pressure and flow responsive mechanisms in the hydraulic circuits provided a means of indirectly measuring and reacting to the power supply of the driving means of the hydraulic system. However, the addition of complex and/or numerous valving and hydraulic mechanisms to the circuits not only increases the cost of the system, but also can make precise and accurate control of the pressure and flow supply more difficult to manage and predict.
Some of the pump control systems have been adapted specifically to tractors or refuse equipment where fluctuating hydraulic needs are common and are further complicated because the drive speed of the pump(s) varies with the speed of the tractor or refuse truck. Often the demands on the hydraulic system are greatest when the engine speed is at its lowest because the tractor or refuse truck is at a standstill. In refuse trucks for example, it has been common to have a single fixed displacement pump to provide for the needs of the hydraulically operated packer. The packer requires a certain level of flow to function adequately and the pump must be run at high speeds to provide that flow. This requires the operator to speed up the engine of the refuse truck to drive the pump at the required speed even if the truck is at a standstill and the horsepower requirements are low. This is normally the case on a front or side loading truck where the refuse is pushed into an empty body. Pressures are low so required horsepower is also low. Some refuse trucks are equipped with variable displacement pumps to handle the changing needs of the hydraulic system and adapt to varying engine speed, but typically there are only a few operating modes and the capabilities of nearly infinite adjustment is deemed too expensive and unnecessary. Further, these types of systems require a more sophisticated mechanic to be able to troubleshoot and repair them. For example, packers and loaders are used on a refuse truck when the truck is stopped and the packer is used when the truck is moving between stops, but there are not commonly many other distinctive modes of operation. A few different operating modes of the hydraulic system would address all of the requirements for variability.
Another variable flow requirement typical of refuse packers is introduced with the inclusion of the telescopic cylinders that are used to drive the packer on front and side loader type refuse trucks. As the packer compresses the refuse, the telescopic cylinder extends and the demand for fluid flow is high due to the relatively large bore and considerable length of the telescopic cylinders used in this application. In addition, the pressure demand is at a high particularly at the end of the packing cycle when the refuse body is almost full. In this condition, the telescopic cylinders extend so as to sufficiently compress or pack the refuse. Even when the body is full, this only happens at the very end of the packing cycle. The first part of the cycle is used to sweep the material toward the body. This uses very little pressure. As the packer returns to its starting position the telescopic cylinder retracts and only a small fraction of the extension flow rate is needed to provide acceptable retracting rates given that the hydraulic fluid now acts on the rod end of the cylinder where most of the volume is occupied by the telescopic rods. The volume ratio for equal extension and retracting speeds can be 4:1 or higher in typical telescopic cylinders. In a typical refuse truck a single fixed displacement pump is usually selected that meets the flow rate requirements of the packer cylinder(s) in the extension cycle. As the same high flow rate is applied to retract the telescopic cylinder, the system wants the retracting speed to be 4 times (for a 4:1 volume ratio) the extension speed. This creates a problem in that it is difficult to evacuate all the fluid from the base end of the cylinder fast enough to allow such high-speed cylinder retraction. The large flows cannot be accommodated by standard lines and valving. What typically happens is that the flow out of the base end of the cylinder is therefore limited by these components. The flow going into the rod end side of the telescopic cylinder is therefore also limited. The excess flow must go over the relief valve. This flow goes over the relief valve at system pressure. Often the volume of fluid that passes over the relief valve is considerable. This generates a significant amount of heat. Common attempts to solve the problem include the inclusion of large and expensive dump valves at the base end of the cylinder, but even with those additions it is frequently not practical to allow such high-speed retraction. Many refuse trucks also include automated loading systems and constantly running packers. These also increase the overheating problem in a hydraulic circuit. The constant heat generation as the packer cylinders retract becomes an even more significant problem.
The present invention is directed to overcoming one or more of the problems set forth above.
In accordance with one aspect of the present invention there is provided an improved pump control system for use on refuse trucks, wherein a plurality of fixed displacement pumps are selectively turned xe2x80x9conxe2x80x9d or xe2x80x9coffxe2x80x9d in response to an external signal.
Another aspect provides an improved pump control system that uses an engine speed measuring device.
Still another aspect provides an improved pump control system, wherein a plurality of fixed displacement pumps are selectively turned xe2x80x9conxe2x80x9d or xe2x80x9coffxe2x80x9d in response to the combination of an external signal and an internal signal.
Yet another aspect provides an improved pump control system that uses an engine speed measuring device and a device to measure system pressure.
In accordance with the present invention there is provided a fluid system for use on a refuse truck and including a hydraulic motor which may be a hydraulic ram; a fluid reservoir; a plurality of fixed displacement pumps; drive means operatively connected to the fixed displacement pumps for driving the same; a control valve for selectively directing a flow of fluid to either the reservoir or the hydraulic motor; and a control system having means for determining an external condition and generating a corresponding signal, and means responsive to the signal for switching the control valve.